Thanks.
I'm going for "dimmer-less". Working out the volumes I was just going to "wing it". But roughly the following:
Take 25 litres of strike water at 70C, add to mash tun (7 (uk)gal water cooler tub). Adjust if necessary. Cover, leave alone for 1 hour.
Drain the mash into the kettle. I expect maybe only getting 20 litres back with a reasonable drain time.
There are options here, but adding 5 litres of 70C (pre-boiled) water to the mash tun in multiple "rinse/sparge stages" is usually what I do.
So I would be back to 25 litres boil volume, which in a 30litre boiler is as far as I am willing to go.
Technically I need only 19lt out of the fermentor, so ... as you point out... I either need to skimp on the sparge water, the mash volume or reduce the wort through evapouration towards the target "pitching" volume.
To get 19litres comfortably, without too much waste, I probably need a pitching volume of about 22 litres.
3 litres sounds about right for the boil "with" the dimmer.
The boiler is a tea urn. The elements are rings which cover a good area of the base. I still do get some caramel debris, but it's not bad enough to melt the bottom out of a paint strainer bag, (I used to mash in the kettle in a bag)
Instead the plan is to 100% it up to the protein hot break. Then manually alternate the thermostat to nurse it through the same. When it's clear of foam back to 100% towards the rolling boil. In with the hops and the thermostat I will "fiddle" with to get it as close to "pulse boiling" as possible. Where it boils for a 10 seconds starts to get violent, then cuts out and cools below the boil and repeats. (that or I end up fiddling with it constantly).
If I end up with too MUCH evaporation, I can part cover it with a humidity hood (lid). If I end up with less, I just steep it longer or manual supervise a violent boil phase.
I do have a copper coil wort chiller and protofloc which usually results in pretty clear wort at 25*C after an hour.
The frustrating bit, it works exactly like LithiumIon chargers do! The first 50*C goes in minutes. The last 5*C takes over half an hour on it's own! Need colder tap water!
Nice idea with kegs. I'm still not convinced with the need. If I wanted to go propane for pure power that would limit me to boiling in the garage or outdoors.
EDIT: I have a propane cooker hob. But for some reason, using one of those 7kW burners indoors sounds like it would be a bad idea??